Non-restorative sleep is a key diagnostic feature of the musculoskeletal pain disorder fibromyalgia, and is robustly associated with poor physical functioning, including activity interference. However, the mechanisms through which non-restorative sleep elicits activity interference among individuals with fibromyalgia at the within-person level remain unclear. The present study tested the following three-path mediation model, using data gathered from a 21-day electronic daily diary in 220 individuals with fibromyalgia: previous night non-restorative sleep → morning pain catastrophizing → afternoon pain severity → end-of-day activity interference. Results of multilevel structural equation modeling supported the three-path mediation model. Previous night's non-restorative sleep and morning pain catastrophizing were also directly related to end-of-day activity interference. Previous night non-restorative sleep did not significantly predict afternoon pain severity while controlling for the effect of morning pain catastrophizing. Greater non-restorative sleep during the previous night and a higher level of morning pain catastrophizing appear to serve as risk factors for experiencing greater daily pain and activity interference later in the day. These findings point to the potential utility of targeted interventions that improve both sleep quality and pain catastrophizing to help individuals with chronic pain engage in important daily activities despite experiencing pain.